Blood agar is an enriched solid medium made by supplementing a basal nutrient agar with around 5 percent defibrinated mammalian blood, usually sheep or horse. The added blood supplies growth factors that support fastidious bacteria and also allows observation of haemolytic reactions.
Explanation
A typical blood agar plate consists of tryptic soy agar or another basal medium to which sterile red blood cells are added at about 45 °C so they remain intact. The resulting medium is both enriched and differential. Many pathogens require the iron, proteins and other nutrients present in blood to grow, making blood agar useful for isolating organisms from clinical specimens. At the same time, bacteria that produce haemolysins will lyse the erythrocytes and alter the appearance of the agar around their colonies. Complete lysis of red cells creates a clear zone, termed beta haemolysis. Partial breakdown of haemoglobin gives the agar a greenish discolouration and is known as alpha haemolysis. Lack of haemolysis leaves the agar unchanged (gamma haemolysis). Blood agar also supports growth of non‑haemolytic organisms, so it provides a broad overview of the microbial flora in a specimen.
Hemolytic Patterns and Applications
Observation of haemolysis helps identify organisms. Streptococcus pyogenes and Staphylococcus aureus produce beta haemolysis, creating a clear halo around colonies. Streptococcus pneumoniae and some viridans group streptococci display alpha haemolysis with a green zone. Enterococcus faecalis and Staphylococcus epidermidis are typically gamma haemolytic, showing no change. Laboratory workers incubate blood agar plates at 35–37 °C in a CO2‑enriched atmosphere to recover respiratory pathogens such as Haemophilus influenzae, Neisseria meningitidis and various streptococci. Blood agar is also used to detect haemolytic reactions of Gram‑positive cocci isolated from throat swabs, wound infections or blood cultures. Modified formulations like chocolate agar, where blood is heated and lysed, cater to even more fastidious organisms. When reading plates, the type of haemolysis, colony morphology and other biochemical traits are combined to identify pathogens accurately.
Blood agar remains a fundamental tool in diagnostic microbiology. Its enriched nature allows cultivation of a wide variety of bacteria, while haemolytic patterns provide immediate clues to species identity. Proper preparation and interpretation ensure reliable results.
Related Terms: Hemolysis, Basal medium, Fastidious organism, Streptococcus, Agar